(venv2.7) osm:~$ pip install matplotlib 
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
Collecting matplotlib
  Using cached https://files.pythonhosted.org/packages/1e/20/2032ad99f0dfe0f60970941af36e8d0942d3713f442bb3df37ac35d67358/matplotlib-2.2.4.tar.gz
    ERROR: Command errored out with exit status 1:
     command: /home/osm/venv2.7/bin/python2.7 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-v0wvdH/matplotlib/setup.py'"'"'; __file__='"'"'/tmp/pip-install-v0wvdH/matplotlib/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base pip-egg-info
         cwd: /tmp/pip-install-v0wvdH/matplotlib/
    Complete output (62 lines):
    ============================================================================
    Edit setup.cfg to change the build options
    
    BUILDING MATPLOTLIB
                matplotlib: yes [2.2.4]
                    python: yes [2.7.15+ (default, Nov 27 2018, 23:36:35)  [GCC
                            7.3.0]]
                  platform: yes [linux2]
    
    REQUIRED DEPENDENCIES AND EXTENSIONS
                     numpy: yes [version 1.16.5]
          install_requires: yes [handled by setuptools]
                    libagg: yes [pkg-config information for 'libagg' could not
                            be found. Using local copy.]
----->            freetype: no  [The C/C++ header for freetype2 (ft2build.h)
                            could not be found.  You may need to install the
                            development package.]
----->                 png: no  [The C/C++ header for libpng (png.h) could not
                            be found.  You may need to install the development
                            package.]
                     qhull: yes [pkg-config information for 'libqhull' could not
                            be found. Using local copy.]
    
    OPTIONAL SUBPACKAGES
               sample_data: yes [installing]
                  toolkits: yes [installing]
                     tests: no  [skipping due to configuration]
            toolkits_tests: no  [skipping due to configuration]
    
    OPTIONAL BACKEND EXTENSIONS
                    macosx: no  [Mac OS-X only]
                    qt5agg: no  [PySide2 not found; PyQt5 not found]
                    qt4agg: no  [PySide not found; PyQt4 not found]
                   gtk3agg: no  [Requires pygobject to be installed.]
                 gtk3cairo: no  [Requires cairocffi or pycairo to be installed.]
                    gtkagg: no  [Requires pygtk]
                     tkagg: yes [installing; run-time loading from Python Tcl /
                            Tk]
                     wxagg: no  [requires wxPython]
                       gtk: no  [Requires pygtk]
                       agg: yes [installing]
                     cairo: no  [cairocffi or pycairo not found]
                 windowing: no  [Microsoft Windows only]
    
    OPTIONAL LATEX DEPENDENCIES
                    dvipng: no
               ghostscript: yes [version 9.26]
                     latex: no
                   pdftops: yes [version 0.62.0]
    
    OPTIONAL PACKAGE DATA
                      dlls: no  [skipping due to configuration]
    
    ============================================================================
                            * The following required packages can not be built:
                            * freetype, png
                            * Try installing freetype with `apt-get install
                            * libfreetype6-dev` and pkg-config with `apt-get
                            * install pkg-config`
                            * Try installing png with `apt-get install
                            * libpng12-dev` and pkg-config with `apt-get install
                            * pkg-config`
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

(venv2.7) osm:~$ echo $CPATH
/home/osm/.local/include:
(venv2.7) osm:~$ echo $C_INCLUDE_PATH
/home/osm/.local/include:

(venv2.7) osm:~$ tree /home/osm/.local/include/ -L 1
/home/osm/.local/include/
├── freetype2
├── gsl
├── libpng16
├── ncurses
├── pngconf.h -> libpng16/pngconf.h
├── png.h -> libpng16/png.h
└── pnglibconf.h -> libpng16/pnglibconf.h